Currently, an electron source that undergoes field emission in response to a strong electric field rather than thermionic emission in response to large heat energy as in a cathode ray tube has been under active research from the perspectives of both device and material. An example of such a conventional electron source can be found in C. A. Spindt (U.S. Pat. No. 3,665,241), which discloses a pyramid-shaped metal electron source. As a material of the metal electron source, a refractory metal such as molybdenum is used, for example. The metal electron source is formed in a hole of about 1 μm. According to the experiment on field emission characteristics conducted by C. A. Spindt (C. A. Spindt, IEEE TRANSACTIONS ON ELECTRON DEVICES, 38, 2355 (1991)), the molybdenum metal electron source that was formed in a gate opening of about a 1 μm diameter is capable of producing emission currents of about 90 μA/tip at a gate voltage of 212 V, which is not low for an operating voltage. Further, the metal electron source using such a refractory metal (known as a Spindt type metal electron source) has a high degree of operating vacuum of 1.33×10−7 Pa (10−9 Torr). In addition, the metal electron source has weak resistance against ion bombardment and therefore reliability is poor. These drawbacks have been a serious obstacle, preventing the electron source from being put to actual applications.
Recently discovered by Iijima et al. is a carbon nanotube as a by-product of a fullerene synthesis by carbon arc discharge (S. Iijima, Nature, 354, 56 (1991)). The carbon nanotube, when observed under a transmission electron microscope (TEM), has an encased structure of graphite layers that are coiled cylindrically (Y. Saito, Ultramicroscopy, 73, 1(1998)). Such a carbon nanotube is called a multi-walled carbon nanotube.
As a producing method of another type of carbon nanotube, there is a technique in which an organic material is applied on a freestanding anodic aluminum oxide film, and then the anodic aluminum oxide film (anodic aluminum oxide layer) is dissolved and removed to separate the carbon nanotubes, as disclosed in Japanese Publication for Unexamined Patent Application No. 151207/1996 (Tokukaihei 8-151207). The carbon nanotubes produced by this method have open ends with a diameter of 1 μm or less and a length of about 1 μm to 100 μm.
There has been active research on electron source using such a carbon nanotube. W. A. de Heer et al. has reported that field emission occurs at an electric field intensity of about 10 V/μm with a degree of vacuum of 1.33×10−4 Pa (10−6 Torr) and an emission current (voltage: 25 V/μm) with a current density of 10 mA/cm2 is generated (W. A. de Heer et al., Science, 270, 1179 (1995)). Such a carbon nanotube electron source is realized by providing carbon nanotubes on a casting film. The carbon nanotube electron source of this teaching undergoes emission at a degree of vacuum that is smaller by triple digits or so than the degree of vacuum required for the metal electron source, and has an emission start voltage and an operating voltage that are smaller by at least one digit than those of the metal electron source. These are superior characteristics as the electron source material. Where a carbon nanotube electron source that undergoes emission of a large current at a low voltage is desired, orientation control of carbon nanotubes becomes an important technique.
Orientation control of carbon nanotubes is an important technique to obtain a carbon nanotube electron source that undergoes emission of a large current at a low driving voltage. Japanese Publication for Unexamined Patent Application No. 12124/1998 (Tokukaihei 10-12124) (Japanese Patent No. 3008852) discloses an electron source wherein carbon nanotubes are provided in the pores of the anodic aluminum oxide film and a gate electrode is provided at the opening of the pores. This carbon nanotube electron source has carbon nanotubes that grow from a metal catalyst, a growth point, that is embedded in the pores of the anodic aluminum oxide film. This carbon nanotube electron source is superior in terms of orientation control (order of orientation) of the carbon nanotubes, and therefore has a promising future. Further, this carbon nanotube electron source has improved stability over time of emission current density. The carbon nanotube electron source also has a greatly improved electron source density that is several thousand times greater than that of the conventional Spindt type metal electron source.
As with the foregoing Tokukaihei 10-12124, D. N. Davydov et al. (D. N. Davydov et al., J. Appl. Phys., 86, 3983 (1999)) discloses producing carbon nanotubes whereby growth of carbon nanotubes originates from a metal catalyst that is provided on the bottom of the pores of the anodic aluminum oxide film and thereafter a portion of the anodic aluminum oxide film is removed to obtain carbon nanotubes with exposed tips.
As schematically shown in FIG. 38 and FIG. 39, the carbon nanotubes that are produced by such a method grow upward from the bottom of the pores of an anodic aluminum oxide film 35 which is provided on an aluminum substrate 30. The carbon nanotubes grow into two different shapes depending on the growth time (extent of growth). That is, when growth of the carbon nanotubes is stopped before it reaches a sufficient level, carbon nanatubes 38 (oriented carbon nanotubes) that are formed in parallel in the pores of the anodic aluminum oxide film 35 are obtained, as schematically shown in FIG. 38. On the other hand, by allowing sufficient growth, carbon nanatubes 36 (random carbon nanotubes) that are interwound randomly on the anodic aluminum oxide film 35 are obtained, as schematically shown in FIG. 39. According to the study done by D. N. Davydov et al., the emission start electric field intensities of these two types of carbon nanotubes were different; 30 V/μm to 45 V/μm for the oriented carbon nanotubes and 3 V/μm to 4 V/μm for the random carbon nanotubes (D. N. Davydov et al., J. Appl. Phys., 86, 3983 (1999)).
However, the conventional electron source using the carbon nanotubes, while it requires a lower operating voltage (device driving voltage: applied voltage at which a practical emission current density (about 10 mA/cm2) is obtained) than the conventional Spindt type metal electron source, still requires a driving voltage of several hundred volts, which is still high. This is because the emission start electric field intensity or operating electric field intensity of the conventional carbon nanotubes (electric field intensity required to obtain a practical emission current density (about 10 mA/cm2)) is not low enough (e.g., carbon nanotubes formed by a conventional arc discharge method have an emission start electric field intensity of 10 V/μm and an operating electric field intensity of 25 V/μm). Insufficiently low driving voltages have put restrictions on drivers or device structures. Thus, there is a need to further reduce emission start electric field intensity and operating electric field intensity.
The present invention was made in view of the foregoing problem and an object of the present invention is to provide carbon nanotubes that require less emission start electric field intensity and less operating electric field intensity, and to provide an electron source, using such carbon nanotubes, that can be driven at a lower voltage. A further object of the present invention is to provide a lower power consuming display that uses the carbon nanotubes in the electron source.
In the producing method of a carbon nanotube electron source using a metal catalyst as disclosed in D. N. Davydov et al., varying manufacture conditions bring about non-uniformity in the shape of carbon nanotubes. That is, the carbon nanotube electron source produced by this method has poor emission uniformity. Such poor emission uniformity becomes particularly prominent when a large device (e.g., a large screen display) is manufactured from the carbon nanotubes produced by this method.
The reason varying manufacture conditions bring about poor emission uniformity is explained below in detail. In the conventional producing method of the electron source, carbon nanotubes grow from the metal catalyst. Therefore, depending of the extent of growth, two different shapes of carbon nanotubes; the oriented carbon nanatubes 38 as shown in FIG. 38 and the random carbon nanatubes 36 as shown in FIG. 39 are produced. Thus, when carbon nanotubes that are formed by the foregoing conventional method are used in an electron source device, for example, such as a display as exemplified by a FED (field emission display), there are cases where varying manufacture conditions cause the oriented carbon nanotubes and the random carbon nanotubes to coexist. The emission start electric field intensities of these two different types of carbon nanotubes, 30 V/μm to 45 V/μm for the former and 3 V/μm to 4 V/μm for the latter, are greatly different. The coexistence of the oriented carbon nanotubes and the random carbon nanotubes in the electron source device has a detrimental effect on uniformity of emission characteristics and causes various problems such as display flicker. The cause of this coexistence of the oriented carbon nanotubes and the random carbon nanotubes resides in a growth mechanism of the carbon nanotubes. Specifically, it is known to be caused by the growth of carbon nanotubes that originate from a transition metal catalyst such as nickel, iron, and cobalt.
The producing method of an electron source disclosed in Tokukaihei 10-12124 also forms carbon nanotubes using a metal catalyst, and therefore has the same problem as the producing method of D. N. Davydov.
Another object of the present invention is to provide a producing method of an electron source having superior uniformity in emission characteristics within a device plane or an emission area (pixels).